Vaccines, What Are They and How Do They Work?
Microbial Vaccines Are Antigen Stimulants to a Host's Immune System
May 6, 2009
Vaccines are prophylactic (disease-preventing) biologicals inoculated into susceptible individuals to protect against potential actual pathogens that may infect a host.
Early History of Vaccination
Vaccines save lives. Vaccines prevent disease. Ultimately, vaccines lower the risks of morbidity (illness) and mortality (death) due to infectious diseases. Earliest vaccination practices can be traced back to ancient China where scarification (roughing of the skin with pustule material) produced immunity and protection against smallpox.
Edward Jenner introduced immunization into England in the 1796 by inoculating people with viruses from sores of milkmaids who had mild skin cowpox. Since milkmaids who had cowpox never contracted smallpox, or only mild forms of smallpox, Jenner proposed there was an obvious protection or immunity. This was a clear example of cross-protection by a virus related to smallpox.
Pasteur used extracts of dried spinal cords from rabbits with the rabies to immunize and protect against rabies or hydrophobia. The rabies vaccine protects in an early rabies infection, or as a prophylactic, disease-prevention, measure.
Drs. Emil von Behring and Kitasato showed that immunity to the deadly diphtheria toxin resided in the serum fraction of coagulated blood.
Vaccine Types, Dead and Attenuated Whole Microbes, Virulence Factors and Components, Toxoids
There types of antigens in microbial vaccines include:
- Dead, complete microbes, as in the original typhoid fever vaccine.
- Live, but attenuated (weakened) microbes, BCG vaccine of Calamette-Guerrin for tuberculosis, measles, mumps, chickenpox and vaccinia.
- Modified toxins = toxoids. Diphtheria and tetanus toxoids of the DT immunization.
- Isolated virulence factors, e.g. capsules, walls, membranes, flagella, structural proteins, enzymes, nucleoprotein complexes. Capsular antigen vaccines for Streptococcus pneumoniae and Haemophilus influenzae HiB vaccine;
Vaccines May Consist of a Single Epitope or Multiple Epitopes
A vaccine antigen may have one or more epitopes, i.e. chemical groups that stimulate cellular and antibody responses. Sometimes, there is only a single kind of antigen. In many cases there may be several, different or mixed antigens. Each category of vaccine above often consists of multiple epitopes that stimulate specific immune responses for each epitope. .
Vaccines Are Introduced Into Hosts To Start Immunization Process
Immunizations may be given:
- intradermally, injected into the skin.
- orally, by ingestion.
- inhaled, breathed in.
- subcutaneously, under the skin and into the lipid layer.
- intraperitoneally, into the membrane or sac surrounding the intestines.
The route of immunization and type vaccine is determined by which works best in experimental or model animal studies and human tests and trials. Eventually, the vaccine is approved for routine medical use.
Antigens in a particular vaccine are recognized by scavenger white blood cells termed macrophages.
- Macrophages phagocytose (eat ,ingest) and process antigens to small molecules.
- Processed small molecules migrate to the macrophage's outer membrane.
- T-helper cells (CD4 lymphocytes) recognize the antigen as foreign. They send chemical signals to recruit and mobilize other white cells, B-lymphocytes to make antibody and killer lymphocytes.
- B-cells produce antibody, multiply and produce more antibody. This remembering response is called the anamnestic antibody or booster response.
- B-cell clones are very specific. They recognize only one feature (epitope) of the virus The clones continue to react and inactivate viral antigen until it is gone. Certain B-cells will live and retain memory of this encounter.
- In a natural infection the immune system recognizes and reacts with that antigen. The macrophages process the antigen, as they did with the vaccine, antibody is produced and lymphocytes activated.
Vaccines and vaccination are among medicine's greatest discoveries, since immune systems can be trained to remember, process and defend against virulent and deadly microbes.
Sources
Brooks, G.F., J.S. Butel and S. A. Moore. 2004. Medical Microbiology. 23rd ed., Lange Medical Books, McGraw-Hill, New York. 818pp
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